1. Field of Invention
This present invention generally relates to medical devices and more particularly to implantable devices for monitoring internal pressure, e.g., intracranial pressure, of a living being.
2. Description of Related Art
Implantable sensors are important diagnostic devices which help measure physiological parameters that are difficult or even impossible to measure noninvasively. However, implantable devices pose several problems for the designer. They have to be biocompatible, so they do not harm the patient over a long or short term, and they cannot trigger physiological or patho-physiological reactions (e.g., immunological reactions) which can compromise their ability to perform measurements.
Another set of problems stems from engineering requirements. The stability requirements for the implantable sensor are more strict that those for the noninvasive devices since they cannot be calibrated at will, or at least, the calibration process is usually more challenging compared to other devices.
The long term implantable pressure sensors carry two inherent problems affecting their stability.
First, short term body temperature fluctuations change the internal temperature, thus changing the internal pressure. This pressure change affects the pressure differential between the internal pressure of the device and the external one (e.g., intracranial pressure, ICP). Another short term factor may include the change in the amount of gas inside the sensor body (e.g., gas absorption due to oxidation or gas release from materials inside the capsule). These types of changes can also add or subtract from forces acting on the transducer by changing forces acting on the membrane separating the inside of the sensor from the external environment. Another source of drift might be related to sensor aging. However, the use of solid state components assures the longevity of the materials.
Second, the natural body responses cause protein deposits on the outside surface of the device, thereby changing the effective stiffness of the membrane. This change in effective stiffness may change the sensitivity of the device or even entirely block the external pressure. This type of problem is usually associated with long term changes.
A typical solution to these problems is to utilize two identical sensors which respond to temperature and aging the same way. One sensor is usually exposed to the measured quantity while the reference one is only exposed to conditions inside the sensor housing. The resulting signal is calculated as a difference between the reference signal and the second sensor. However, this solution has several drawbacks: e.g., the reference pressure in the reference transducer has to be kept constant.
The above-listed problems (assuming that the membrane by itself does not generate any stress on the sensor regardless of the displacement, i.e., an ideal membrane) causes the output-input characteristic of the sensor to shift up or down (see FIG. 1A); or to rotate about certain point changing the slope of the characteristic (FIG. 1B). In particular, plot 1 of FIG. 1A depicts the undisturbed input-output characteristic. Plot 2 depicts the input-output characteristic of the internal pressure (i.e., inside the sensor body) which is lowered. Plot 3 depicts the input-output characteristic if the internal pressure is elevated.
Every sensor carries an inherent risk of drifting with time. While several compensation methods exist for external sensors, the drift problem is accentuated in the case of an implantable sensor. The active element of the sensor (e.g., piezoresistive element or die) changes its properties with time, temperature etc. FIG. 2 depicts the variance of output vs. measured quantity (e.g., pressure) as temperature changes. The lower line 2A in FIG. 2 represents the normal operation curve of the die when operating at a temperature T1. The slope of this line 2A represents the sensitivity of the sensor at that temperature. If the temperature is increased, the piezoresistive die's response to changes in pressure also changes (see upper line 2B in FIG. 2); in particular, the sensitivity changes and also an offset component is introduced. Such factors can be resolved by hardware and, typically, sensor housings are constructed with built-in compensation. However, such solutions increase the size of the sensor and the power consumption.
One of the physiological parameters which is difficult to measure noninvasively is ICP. ICP can be an important parameter in monitoring hydrocephalic patients, or traumatic brain injury (TBI) victims.
Since cerebrospinal fluid is enclosed in a semi closed system (i.e., the skull), the forces exerted by it are counterbalanced by a rigid structure of bones and, to some extent, by a semi rigid structure of the spinal channel. In a mechanical sense, there is no direct link (except for some small vessels which are difficult to utilize due to their anatomical nature) between the cerebrospinal fluid and the external environment. Thus, an implantable sensor outfitted with a reliable means of calibration would be a valuable addition to neurosurgical armamentarium.
U.S. Patent Publication No. 2013/0247644 (Swoboda, et al.), which is owned by the same Assignee as the present application, namely, ICPCheck, Inc. of Bensalem, Pa., discloses a calibration system and method for an implantable pressure sensor having wireless communication capability. The inventions disclosed therein are directed to static calibration systems and methods of the implantable pressure sensor.
However, there remains a need for an implantable pressure sensor having wireless communication capability that can account for these artifacts and provide a more accurate reading of the internal pressure to be measured but which uses dynamic calibration systems and methods.
All references cited herein are incorporated herein by reference in their entireties.